1. Field of the Invention
The invention relates to a voltage multiplier or booster circuit working at low supply voltages. A circuit such as this is used to produce an output voltage from a received supply voltage by the charging and discharging of one or more capacitors consecutively so that the output voltage is greater than the supply voltage. This circuit, which is called a load pump, finds application chiefly in the supplying of capacitive circuits.
2. Discussion of the Related Art
Voltage boosters are used for example in the field of non-volatile memories organized in matrix form in rows and columns, and comprising storage cells made of MOS-type transistors placed at the intersections of the rows and the columns. Voltage boosters are used to produce bias voltages applied to the control gates of these transistors. Indeed, a model may be made of all the control gates of these transistors by means of an equivalent capacitance, these control gates being voltage-controlled and not current-controlled. For a read-only memory comprising several thousands of rows and columns, an equivalent capacitance value of one or more hundreds of picofarads is commonly reached.
In the prior art, there are known voltage booster circuits called Schenkel circuits. This type of circuit has one or more series-connected stages. Each stage has one input and one capacitor, a first terminal of which is connected to the input. This terminal receives a clock signal. The other terminal of this capacitor is connected to an output by means of an insulation diode.
Typically, the clock signal oscillates between a reference potential received at a first input terminal and a supply potential received at a second input terminal (in practice, a ground and a positive voltage VCC of the order of 5 volts for example).
The capacitor of the first stage has its second terminal connected to the second input terminal by means of another diode. Thus, when the clock of the first stage is at the ground potential, the capacitor is charged at the potential of this input terminal, minus losses due to the threshold effect and the substrate effect in the diode (made in practice with a MOS transistor whose control gate is connected to the source). Then, when the clock signal is at the potential VCC, the voltage at the terminals of the capacitor rises to 2 * VCC-Vt, and this capacitor then charges the capacitor of the next stage for which it has been ensured that the clock signal is placed at the ground potential. When the potential of the clock signal goes to the potential VCC, the voltage at the terminals of the capacitor of the second stage then rises to 3 * VCC-2 Vt (transfer diode) etc. Thus, at each stage, the voltage at the terminals of the capacitor increases by VCC-Vt multiplied by one. The last stage gives the output voltage at its output. This output is a capacitive node and a model may be made of the circuit to be supplied by means of a capacitor connected between this output and the ground. By charge transfer, the output potential will gradually rise by ever smaller stages until it reaches the potential (n+1)*(VCC-Vt), with n as an integer, taking n series-connected stages. The speed of this increase will greater or smaller depending on the capacitance values of the capacitor and on the transfer gain resulting therefrom.
This type of circuit has two drawbacks:
firstly, because of the losses due to the insulation diodes mounted between the stages, the value of the output voltage is limited to (n+1)* [VCC-Vt]. Since, in order to restrict power consumption by the integrated circuits, it is desired to develop circuits supplied with lower supply voltages (for example 3 volts), this limitation of the output voltage is a detrimental factor. Indeed, the threshold voltage of a diode is typically equal to 1.2 to 1.5 volts, which is a limitation equivalent to half of the value of the supply voltage at worst, between one stage and the next stage; PA1 secondly, these losses limit the number of charges transferred between each stage, causing an increase in the build-up time of the output voltage.
For example, if is desired to double the supply voltage and thus go from 3 volts to 6 volts, theoretically only one stage is enough. In practice, the output voltage is then limited between 4.5 and 5 volts. It is therefore necessary to add on at least one additional stage, which entails penalties in terms of the amount of space occupied. It is furthermore necessary to take account of the additional space requirement due to the presence of control circuits to produce the clock signal of the additional stages and to coordinate the clock signals with one another.
It is also possible to make the diodes using so-called native transistors, namely non-implanted transistors. These transistors have a low threshold voltage of the order of 0.2 to 0.4 volts. These approach would make it necessary, however, to qualify this type of technology, which is a particularly long and costly process.